1. Field of the Invention.
This invention relates tO power plants for model airplanes and the like and more particularly to a turbine-like power plant for use in model jet aircraft.
2. Description of the Prior Art.
During the days when only reciprocating engines were used for aircraft, a number of small reciprocating type internal combustion engines were developed for powering model aircraft. The engines have been very highly developed so as to provide an ideal engine for small model airplanes driven by conventional propellers. However, with the advent of modern jet aircraft, model builders and model airplane enthusiasts have generated a requirement for a modified reciprocating internal combustion engine which could be installed inside of the fuselage of the model to power scale model jet aircraft either single or multi-engine type.
In response to this requirement, a number of shrouded fan type engines have been developed. A typical configuration for such shrouded fan engines may be found in U.S. Pat. No. Des. 241,360 to Scozzafava which is typical of known prior art engines of this type. A cylindrical duct or shroud is provided having a series of radial struts which support an engine mount concentric with the duct. The engine mount is generally cylindrical, tapering to a small diameter at its rear end. The cylindrical mount is cut away to permit a conventional model internal combustion engine, such as previously used to drive a propeller, to be mounted therein. A multiblade fan is attached to the output shaft of the engine, which is concentric with the shroud, such that the blades are fully enclosed by the duct or shroud.
Although the prior art engines of this type have been used by mounting the entire unit inside the fuselage of the model aircraft and having a jet-like opening at the rear through which the moving stream of air produced by the rotating fan blades can issue, the prior art designs have resulted in a number of problems, many of which are necessarily corrected by modifying the air frame. Among the problems is that of straightening the airflow which will have a spiral slipstream due to the fan rotation. Another problem is the non-uniformity of the slipstream due to problems in providing cooling air to the engine. This problem is compounded by the conventional carburetor designs. In the usual model aircraft engine for propeller driven planes, carburetor barrels, controls and adjustments are horizontally oriented and project outboard from the engine. This is normally done with propeller driven model planes to permit the carburetor controls to project through the cowling for ease of adjustment. When used with a ducted fan engine, a horizontal carburetor construction will result in an interrupted and uneven slipstream since the carburetor is assymmetrical with respect to the engine mount and slipstream.
Another problem present in the prior art engines of this type is that of maintaining maximum efficiency of the fan. For high efficiency, it is necessary to minimize the clearance between the fan tips and the shroud; however, most fans are molded in one piece from plastics which tend to have shrinkage which is not always symmetrical with the results that fans may have non-uniform blade tip clearance which reduces the efficiency thereof. Similarly, when the duct or shroud is molded, for example, such as that of the Scozzafava patent, it is very difficult to obtain and exactly round shroud due to the complexity of the mold and the tendency of the plastic to shrink slightly. Therefore, it is often necessary to build up or trim the shrouds by hand to compensate for such deviations which adds to the cost and complexity of the power plant. The assembly of the engine mount within the shroud can cause distortion.
Therefore, there is a need for a modern fan-type model aircraft power plant designed specifically for use in model jet aircraft in which the spiraling slipstream can be straightened accurately, having a carburetor which will not interfere with the airflow, which can be completely faired to the airstream, which has all engine adjustments outside the airflow, and which has a fan and shroud assembly in which all blades are identical and have uniform clearance with the shroud. The shroud design should allow the airstream to flow around the engine cylinder without appreciable pressure losses.